Various well-known medical techniques for the treatment of malignancies involve the use of radiation. Radiation sources, for example medical linear accelerators, are typically used to generate radiation to a specific target area of a patient's body. Use of appropriate dosimetry insures the application of proper doses of radiation to the malignant areas and is of utmost importance. When applied, the radiation produces an ionizing effect on the malignant tissue, thereby destroying the malignant cells. So long as the dosimetry of applied radiation is properly monitored, the malignancy may be treated without detriment to the surrounding healthy tissue. Accelerators may be utilized, each of which have varying characteristics and output levels. The most common type of accelerator produces pulse radiation, wherein the output has the shape of a rectangular beam with a cross-sectional area which is typically between 16 and 1600 square centimeters. Rectangular or square shapes are often changed to any desired shape using lead or cerrobend blocks, using molds and casting procedures. More advanced accelerators use multileaf collimators. Other accelerators are continuous or nonpulsed such as cobalt radiation machines; and accelerators that utilize a swept electron beam, which sweep a very narrow electron beam across the treatment field by means of varying electromagnetic fields.
To ensure proper dosimetry, linear accelerators used for the treatment of malignancies must be calibrated. Both the electron and photon radiation must be appropriately measured and correlated to the particular device. The skilled practitioner must insure that both the intensity and duration of the radiation treatment is carefully calculated and administered so as to produce the therapeutic result desired while maintaining the safety of the patient. Parameters such as flatness, symmetry, radiation and light field alignment are typically determined. The use of too much radiation may, in fact, cause side effects and allow destructive effects to occur to the surrounding tissue. Use of an insufficient amount of radiation will not deliver a dose that is effective to eradicate the malignancy. Thus, it is important to be able to determine the exact amount of radiation that will be produced by a particular machine and the manner in which that radiation will be distributed within the patient's body. In order to produce an accurate assessment of the radiation received by the patient, some type of pattern or map of the radiation at varying positions within the patient's body must be produced. These profiles correlate 1) the variation of dose with depth in water generating percent depth dose profiles and 2) the variation of dose across a plane perpendicular to the radiation source generating the cross beam profiles. These particular measurements of cross beam profiles are of particular concern in the present invention. Although useful for other analyses, the variation in the beam uniformity of the radiation field regardless of gantry orientation is the main purpose of this device.
One existing system for measuring the radiation that is produced by medical linear accelerators utilizes a large tank on the order of 50.times.50.times.50 cm filled with water. A group of computer controlled motors move the radiation detector through a series of pre-programmed steps beneath the water's surface. Since the density of the human body closely approximates that of water, the water-filled tank provides an appropriate medium for creating a simulation of both the distribution and the intensity of radiation which would likely occur within the patient's body. The aforementioned tank is commonly referred to as a water phantom. The radiation produced by the linear accelerator will be directed into the water in the phantom tank at which point the intensity of the radiation at varying depths and positions within the water can be measured with the radiation detector. As the radiation penetrates the water the direct or primary beam is scattered by the water, in much the same way as when the radiation beam impinges upon the human patient. Both the scattered radiation as well as the primary radiation are detected by the ion-chamber, which is part of the radiation detector. The ion-chamber is essentially an open air capacitor which produces an electrical current that corresponds to the number of ions produced within its volume. The detector is lowered to a measurement point within the phantom tank and measurements are taken over a particular time period. The detector can then be moved to another measurement point where measurements are taken as the detector is held in the second position. At each measuring point a statistically significant number of samples are taken while the detector is held stationary.
Several prior art devices are known to teach systems for ascertaining the suitable dosimetry of a particular accelerator along with methods for their use.
U.S. Pat. No. 5,621,214, issued Apr. 15, 1997, to Sofield, is directed to a radiation beam scanner system which employs a peak detection methodology. Except for the peak detection, this system operates like any other conventional scanning system, using two ion chamber detectors, a signal and a reference detector. In use, the reference detector remains stationary at some point within the beam while the signal detector is moved continuously by the use of electrical stepper motors.
U.S. Pat. No. 4,988,866, issued Jan. 29, 1991, to Westerlund, is directed toward a measuring device for checking of radiation fields from treatment machines for radiotherapy. This device comprises a measuring block that contains radiation detectors arranged beneath a cover plate and provided with field marking lines and an energy filter. The detectors are connected to a read out unit for signal processing and presentation of measurement values. Westerlund arranges the dose monitoring calibration detectors in a particular geometric pattern to determine homogeneity of the radiation field. In use, the measuring device is able to simultaneously check the totality of radiation emitted by a single source of radiation at varying positions within the measuring block. Although Westerlund's does not use a water phantom, his device is nevertheless limited in that all of the ionization detectors are in one plane. This does not yield an appropriate threedimensional assessment of the combination of scattering and direct radiation which would normally impinge the human body undergoing radiation treatment. Thus, accurate dosimetry in a real-life scenario could not be readily ascertained by the use of the Westerlund device.
U.S. Pat. No. 5,006,714, issued Apr. 9, 1991, to Attix utilizes a particular type of scintillator dosimetry probe which is manufactured from a material that approximates water or muscle tissue in atomic number and electron density. Attix indicates that the use of such a detector minimizes perturbations in a phantom water tank. While recognizing the use of a polymer material which is similar to water or muscle tissue in atomic number and electron density, Attix nevertheless requires the use of a cumbersome phantom water tank.
Additionally, there is an apparatus called a Wellhofer "bottle-ship" which utilizes a smaller volume of water than the conventional water phantom. The Wellhofer device still utilizes a timing belt and motor combination to move the detector through the water, thus requiring a long initial set-up time. Lastly, the Wellhofer device still operates on the principle of moving the detector through the phantom body, while the instant device moves a substantially smaller (15.times.15.times.15 cm) plastic phantom body through the radiation field.
Thus, there exists a need for a device that is capable of quickly and accurately detecting both scattering and direct radiation components from radiation devices without requiring the use of a large and cumbersome water phantom.